Speaker grills on or off?

No question. OFF,unless you have to have them on because of children or cats...

I always have the grills off when listening to music. I can hear a clearer difference.

Grill's on or off? It boils down to how the designer miked the speaker in a anechoic chamber to achieve a perfectly flat response. Some designer's will mike and adjust the crossover with the grills on and some with the grills off.
Call the speaker company you plan on buying from and ask the engineer's if their speakers are adjusted in a chamber with the grills off or on. This is why some speakers sound better with the grills off and others with the grills on.

Well if you own Wilsons its no problem, they just fall off.......Why a speaker that expensive has no real good system to hold the grills on is crazy......So be it !!     autospec

I'm lucky. My speakers use Accuton drivers which have a permanently affixed metal grill. I get the best of both worlds: They don't interfere with the music and I can clearly see the drivers.

All the best,
Nonoise